Oxygen Analyses of Minerals and Oxides TALESHWAR SHARM.A and ROBERT N. CLAYTON Enrico Fermi Institute for Nuclear Studies and Department of Chemistry, University of Chicago, Chicago, 111.
b Bromine pentafluoride (BrFs) has been used to extract oxygen quantitatively from the common rock-forming minerals. By a simple volumetric measurement the oxygen content of rocks and minerals can b e determined with an accuracy at least as good as that attainable b y conventional techniques on the other major elements. The method of extraction is simple and procedure reqluires only small samples, on the order of 20 to 50 mg. The direct determinotion of oxygen can b e useful in solving some of the structural problems of minerals such as amphiboles, micas, and others where an excess or deficiency of oxygen is reported on the basis of cation analysis.
T
HE
CHEMICAL
COMPOSITION
Of
silicate minerals and rocks is usually determined 11y measuring the metal ion concentrations and expressing the composition in terms of oxides based on assumed oxidation states of the metals. Anion analyses are sometimes done for fluoride, chl.oride, carbonate, and phosphate, which are usually present tc+ the extent, of less than 1% in most rocks and minerals. An independent determinat.ion of oxygen is dcsirable for several reasons: as a check on the accuracy of the cation analyses, Imticularly for difficult' elements such as H, Si, XI; to avoid the necessity of assuming the oxidati'on states of Fe, iVn> Ti, and other transition metals; and to detect nonstoichiometry and defec,t structures. As the major anionic constituent in most minerals is oxygen, the main object of this research work was to devise a procedure for the direct determination of oxygen in minerals and rocks. The minerals analyzed in this work include quartz, orthoclase, wollastonite, muscovite, amphiboles, magnetite, hematite, olivine, kyanite, garnet, epidote, and serpentine. In several recent studies of the abundances of stable isotopes of oxygen, chemical techniques have been dereloped for the extraction of oxygen from oxides and silicates. Two general types of analytical methods have been used the carbon reduction method ( 1 , 5 ) 6 , 8, 12, 25) and the fluorine or halogen fluoride method ( 2 , 7 , 11, IS, 15-17, 21, 22). These methods deal wit,h the oxygen
energy gamma radiation (7.1 and 6.1 m.e.17.) emitted by the 7.4-second isotope N16. A literature survey based on these papers indicates three major difficulties: the interference of the primary reaction F19 ( n , a ) N I 6 ; the interference of the reaction 1311(n,p)B11 (6.8 m.e.v. gamma) ; and the short half life (7.4 second) of the gamma radiation of N16 produced in the reaction 0I6(n,p) "6. T'olborth and Banta (27,68) have reported quantitative determination of oxygen in HzO, TiOz, ;\l203, S O 2 , and rocks G-1 and W-l.
extraction for the measurement of natural isotopic abundances, where quantitative oxygen yields are necessary. There are several other methods for the extraction of oxygen from oxygenbearing compounds, hut these methods give incomplete oxygen yields and cancot be used satisfactorily for measurement of isotope abundance variations. These can be used for isotopic tracer work where quantitative oxygen yields are not required. Among such methods mention may be made of the Hg(C?r')z method of Hager and Killiams (10) and the guanidine hydrochloride method of Boyer et al. ( 3 ) . A review of the methods used for tracer work is given by Taube (20). X full discussion of the analytical methods have been given by Clayton and Mayeda ( 7 ) , and Sharma (13). Seutron activation techniques have recently been developed for the determination of oxygen. Neutron activation analyses for oxygen have been used by Veal and Cook (2.9)) Steele and Meinke (IQ), Stallwood et al. ( l a ) , Volborth and Banta (27, 28) and Volborth (26). All of these methods use the reaction 0l6 ( n , p ) P and the high
Table I.
60637
EXPERIMENTAL
Apparatus, Reagent, and Analytical Procedure. The apparatus, reagent', and analytical procedures are as described by Clayton and Mayeda ( 7 ) . Some changes have been made in loading the sample and the measurement of the yield. The sample is put into a nickel cup, the nickel cup is held by means of a nickel wire, and the cull along with the wire is put inside the reaction tube. This method of sample handling ensures complete removal of reaction products from the reaction tube to avoid contamination of sub-
Oxygen Analyses of Compounds of Known Chemical Formula
Calcd. oxygen content, pmole/mg. 16 65 16 65 14 38 12 91
Sample Si02 (Quartz)
15 12 8 14 7 8 13
Mg o
Measured oxygen content, pmole/mg. 16 72 16 64 14 40 12 73 15 12 8 14 7
06
40 91 71 04O 64b
03 35
S o . of
analyses 9 20
Std. dev., pmole/mg. 0 17 0 10
6
0 06
22
0 11
15 2
0 0 0 0 0 0 0
10 01 06 02
94 2 CaO 76 2 TiOz 24 3 05 Na2S04 8 61 3 05 CaSOd. l / ? H,O (hlg, Fe);;Si04 (Olivine) 8Sc 13 79 2 01 (Fe, Mg, Ca)3(A1, Fe)z(Si04)3 (Garnet) 13 32d 13 43 3 0 09 Cap(Al, Fe)Al~SiO4Si,0,(OH) 14 06 4 0 09 13 82e ( E pidote) 15.04 3 0 06 15 44 AlZSiOj (Kyanite) a Calcd. for 1 mole O2 per mole NazS04. (When sulfates are reacted with the reagent, half of the sulfate oxygen is liberated as 0 2 , the rest presumably going to S O Z Fwhich ~ 2BrF5 = CaF2 Q02F2 0 2 is not collected in the analytical procedure CaSOr 2BrF3.) b Calcd. for 5 / j mole O2 per mole CaS04, '/z HzO. c Calcd. from MglSi04 92c/,, FefSi04 -8%. d Calcd. from Fe3A12(Si04)3= 407c, Mg3Alz(Si04)3 = 43c/c, Ca3AlZ(Si,O4)3 = 14Cj,, = 2%. Ca~Fe~(Si04)3 Calcd. from CazAIAlzSio,Si,(~i((~H) = 40%, Ca2Feill2SiO4S1~Oi(OH) = 607,.
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+
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VOL. 36,
NO. 10, SEPTEMBER 1964
2001
sequent samples. The wire is put inside the reaction tube. The osygen yield is measured in a calibrated constant volume manometer designed after German ( 9 ) . The manometer is enclosed in a plastic bos to have constant temperature during osygen measurement. The manometer is read by means of a cathetometer with an accuracy of *0.2%. The inanometer mas calibrated by comparison with a known volume. Thus the analytical result is absolute in the senhe that no calibration is carried out using mineral samples.
fluoride a t 100” C. to given quantitative yields of carbon dioxide and oxygen according to the equation (4) : CaC03
+ 13rFs-
ACKNOWLEDGMENT
CaFz
+ BrF3 +
co2 + ‘/z
+ 313rF6+. CaFz + 3BrF3 + CF4
RESULTS
02
CaS04
+ 213rF5
+
CaFz
+ BrF8 + SWz
+
0 2
This fact was also noted by Sheft, Martin, and Katz (16). CONCLUSIONS
I n comparison with activation techniques, this procedure has the advantage of requiring only small samples, on the order of 20 to 50 mg., whereas the procedure of Volborth and Banta (25) requires 2 grams of sample. This difference would be of considerable importance in the determination of the composition of individual minerals, since the separation of 2 grams of a pure mineral is often not feasible. The princilial advantage of the activation method is its speed, which makes it the
Oxygen Analysis for Rocks and Minerals with Major Element Analysis
Calcd. hleasured osygen content, oxygen content, pmole/mg . pmole/mg.
Rock and mineral Hornblende Hornblende Hornblende I
